1975 F-250 4WD, 4 wheel drums.

The master cylinder cover gasket was leaking; I could not find a gasket anywhere, but ended up buying a brand new MC (Raybestos MC39468) from Rock Auto. It was $16, so I figured I couldn't miss.

When the MC arrived, I noticed it looked different than my original, and I couldn't just rob the gasket. Ports were the same size in the new one (they are different sizes in the old one), although it came with adapters. The new unit was also a little wider. The booster bolt pattern, and bore diameter were the same, though.

I bench bled and installed the MC. Bled all 4 corners about 5-6 times.

Now the issue is that the pedal goes down quickly, and it does not return like it should. The brakes work, but the pedal goes right down, and then doesn't come back up like it used to.

Do I have the wrong MC? Could this be the MC for a disc brake truck? Rock auto specifically mentions front drum brakes in the part application.

Do I still have air in the system?

Is the booster shot? The booster is not that old, and the check valve is good, and the vacuum source is manifold vacuum. I have done the typical booster checks, and I believe it is working correctly.

Would air in the system cause the pedal to not return correctly?

Any and all help would be appreciated

 

More then likely because of the difference in the Master cylinders the push rod will need to be adjusted ..